Chamber for the high-frequency heating of a conducting medium

ABSTRACT

The chamber for a high-frequency heating of a conducting medium, and in particular a plasma, is formed of assembled sections each comprising joining means and a shell section formed of leak-tight material. The chamber essentially comprises at least one socalled composite section which contains a high-frequency winding within the shell section and is provided with a plurality of metallic rods forming a cage section between the conducting medium and the high-frequency winding, the rods being arranged such that substantially all the particles constituting the conducting medium can bombard only a metallic surface.

United States Patent 1191 Consoli et al.

]rN0V. 20, 1973 CHAMBER FOR THE HIGH-FREQUENCY HEATING OF A CONDUCTING MEDIUM [75] Inventors: Terenzio Consoli, Meylan, France;

Shigeo Nagao, Sendai, Japan [73] Assignee: CommissariatA LEnergie Atomique, Paris, France [22] Filed: Nov. 15, 1972 21 Appl. No.: 306,912

[30] Foreign Application Priority Data Nov. 22, 1971 France 7141741 [52] US. Cl. 219/10.65, 219/10.79, 310/11 [51] Int. Cl. H051 35/06 [58] Field of Search 219/1065, 10.79; 310/11; 13/1 [56] References Cited UNITED STATES PATENTS 3,415,968 12/1968 Watson 219/1065 3,333,123 7/1967 Baumann 310/11 3,596,117 7/1971 Andresen 310/11 FOREIGN PATENTS OR APPLICATIONS 231,388 12/1958 Australia 219/1065 Primary ExaminerBruce A. Reynolds Attorney-Robert D. Flynn et al.

[5 7 ABSTRACT The chamber for a high-frequency heating of a conducting medium, and in particular a plasma, is formed of assembled sections each comprising joining means and a shell section formed of leak-tight material. The chamber essentially comprises at least one so-called composite section which contains a high-frequency winding within the shell section and is provided with a plurality of metallic rods forming a cage section between the conducting medium and the high-frequency winding, the rods being arranged such that substantially all the particles constituting the conducting medium can bombard only a metallic surface.

15 Claims, 6 Drawing Figures 7/1959 France 219/1079 PATENTEDnnv 20 ms 3.774.001

SHEET 1 [1F 4 PATENTEI] NOV 20 I975 sum 3 0F 4 sbi PATENTEDNDY 20 1975 SHEET k BF 4 CHAMBER FOR THE HIGH-FREQUENCY HEATING OF A CONDUCTING MEDIUM This invention relates to a chamber for highfrequency heating of a conducting medium.

The invention finds application in the construction of devices for heating metallic vapors, for example, or creating plasma which is capable of producing thermonuclear fusion reactions.

It is known that chambers of this type are usually constituted by a leak-tight shell (either linear or toroidal) which is associated with high-frequency heating means. In the case in which these means for heating the conducting medium are constituted by high-frequency field coils, said heating is carried out either through the chamber when this latter is formed of insulating material which is permeable to the high frequency or by forming a break in the structure when this latter is of metal and by interposing sections which are specially designed for heating.

The use of metal for the purpose of forming certain portions of the chamber offers the following advantages (1) metal has good resistance of ion bombardment and to bombardment by neutral atoms derived from the plasma (2) metal is well suited to the production of a high initial vacuum since it can be carefully degassed and (3) the chamber is not contaminated during operation by dissociation of the metal under the action of the ion bombardment. On the other hand, these structures have the obvious disadvantage of being impermeable to the high-frequency field. Although the use of insulating material for the construction of the shell of the chamber does offer an advantage in that it permits the use of a high-frequency winding located outside the structure, it nevertheless results in the appearance of impurities caused by the decomposition of said material under the action of the ion bombardment.

The object of the present invention is to provide a chamber which retains the advantages of the metallic structure without being subject to any of its disadvantages.

SUMMARY OF THE INVENTION In more precise terms, the present invention includes a chamber for the high-frequency heating of a conducting medium and in particular a plasma, said chamber being formed of assembled sections each comprising joining means and a shell section formed of leak-tight material. The chamber essentially comprises at least one so-called composite section which contains a highfrequency winding within said shell section and is provided with a plurality of metallic rods forming a cage section between said conducting medium and said high-frequency winding, the rods being arranged such that substantially all the particles constituting said conducting medium can bombard only a metallic surface.

In a preferred embodiment, the metallic rods which form the cage section are hollow tubes which carry an electric current as well as a circulation of cooling fluid. In this case, the electrical connections of the metallic tubes are such that the current which passes through these tubes produces within the plasma-formation zone a magnetic guard field which reduces the bombardment of the shell.

A better understanding of the invention will be obtained from the following description in which two alternative embodiments are given for the case in which FIG. 1 is a diagram of a toroidal chamber formed of assembled sections FIG. 2 is a diagrammatic view in transverse crosssection showing a composite section for the case in which the cage is formed of two rows of tubes, in two relative positions of one row with respect to the other FIG. 3 is a developed diagram of series connection of the tubes when these latter are supplied with electric current, for the case in the which the cage is formed of a single row of metallic tubes;

FIG. 4 is a developed longitudinal cross-sectional view of a composite section in which the cage is made up of two rows of tubes FIG. 5 illustrates a particular case in which the metallic tubes are wound in a helix and FIG. 6 shows a few examples of transverse crosssectional configurations of tubes.

As FIGS. 2 to 6 represent either transverse crosssections or developed views, these figures can similarly represent the case in which the chamber is linear and not toroidal.

In FIG. 1, there is shown diagrammatically a toroidal chamber for the formation of a plasma in the vicinity of the circle 1, namely the geometrical locus of the center of the circle which generates the torus. Said chamber is formed of eight sections which are numbered from I to VIII. Section I, which is designed in accordance with the present invention, comprises a shell 2 and joining means 4 and 6 which are located at the extremities of the section. Section H, which can be considered as being representative of a particular case of the prior art, is made up of a shell 8 within which is placed a metallic tube 10 having a bellows-type wall section II is also provided with joining means 12 and 14. The transverse cross-section of the shells is shown at the top of the figure on the right. The complete toroidal chamber is formed of a series of sections which are not necessarily identical but comprises at least one section of type I. In one preferred embodiment, all the sections are of type I. In another embodiment, one-half of the sections are of type I, said sections being arranged in alternate relation to those of traditional type for example, the sections III, V and VII are in accordance with the invention whereas the sections II, IV, VI and VIIIare in accordance with the prior art. In FIG. 1, the magnetic confinement means are represented diagrammatically by a few solenoid coils 15 which surround the torus. The pumping means which serve to pump neutral atoms are not illustrated.

In FIG. 2, which is a diagrammatic transverse crosssectional view of a composite section, the highfrequency winding 32 is placed within the metallic shell 5 a first so-called inner row of metallic tubes 36 is located on a torus, the generating circle of which has a radius r,, and a second so-called outer row of metallic tubes 34 is located on a torus whose generating circle has a radius r,. There are shown in the left half of FIG. 4 the two rows of tubes in the case in which the conductors of the outer row are located in the geometrical shadow of the conductors of the outer row or, in other words, in which it is assumed that an observer stationed at the center of the cross-section would see the tubes in superimposition. There is shown in the right half of the figure a different arrangement such that the plasma sees the two rows in intercalated relation. The metallic tubes 34 of the outer row and the tubes 36 of the inner row have a circular transverse cross-section in FIG. 4. It remains wholly apparent, however, that the invention is not limited in any sense to this particular case but extends to all possible alternative arrangements.

In a preferred embodiment of the invention, the metallic tubes are hollow, so that it is possible to circulate within these latter a cooling fluid (such as water, gas or liquid nitrogen). The object of this cooling is to reduce degassing during operation and to lower the resistivity of the metal forming the tubes.

The tubes 34 and 36 can carry an electric current in order to produce a magnetic guard field in the vicinity of the tubes of the cage. In the right portion of FIG. 2, there is shown a particular case of construction in which only the tubes of the inner row carry an electric current. These tubes are connected electrically in series as shown in FIG. 3.

In FIG. 3, which is a developed diagram of the toroidal cage section, section I is shown as having only ten metallic tubes 26 for the sake of enhanced clarity of the drawing. These tubes are connected electrically in series by means of the five bridge elements 28 at the left extremity and the four bridge elements 30 at the right extremity. The current input and output connections are designated respectively by the references 22 and 24. The direction of the current within each tube 26 is indicated by an arrow.

The magnetic field produced by the circulation of these currents is of the multipolar type, with zero amplitude within the greater part of the internal volume of the cage except in the immediate vicinity of the walls, where it increases exponentially. This type of field reduces ion bombardment and performs a function which is similar to that of the limiter of the prior art but nevertheless offers an additional advantage in that this diaphragm function is not attended by any danger of contamination.

The mode of connection of adjacent sections II, III, etc., must be the same as that of the section I as shown in FIG. 3 this being the case, the magnetic fields produced by the flow of current within the bridge elements 28 and 30 are mutually compensated from one section to the other.

A parallel electrical connection of the tubes 26 can readily be conceived and therefore does not require diagrammatic illustration. In this case, the magnetic field produced is practically zero up to the proximity of the conductors 26, which is again equivalent to a diaphragm.

In the right-hand portion of FIG. 2, there are shown around two particular conductors the magnetic field lines 38 and 42 which characterize the multipolar magnetic field obtained. In this particular case, the metallic tubes 34 of the outer row do not carry an electric current and areintercalated with respect to the tubes 36 of the inner row. Taking into account the shape of the field lines 38 and 42, it can be said that the tubes of the outer row are in the magnetic shadow of the inner conductors or in other words that the magnetic leak field produced by the tubes carrying opposite currents is intercepted by the rods which constitute the outer row.

In the left-hand portion of FIG. 2, there is shown a situation in which the tubes of both rows carry an electric current. In each row, a tube carries a current of opposite direction to the current which passes through adjacent tubes. Moreover, the adjacent tubes forming part of two different rows carry currents having the same direction. The result thereby achieved is that, in the plasma-formation zone, the magnetic field is still of the multi-polar type but that, behind the tubes of the inner row, the field lines represented diagrammatically by the references 46 and 48 are very different from the field lines 38 and 42. By means of this arrangement, the ions which pass out of the plasma are guided by the magnetic field lines such as the lines 46 and 48. In consequence, in order to increase the effectiveness of protection of the shell 5 and of the winding 32 against this ion bombardment, there can be placed between the inner and outer rows a third row of tubes 50 located on a torus whose generating circle has a radius r the tubes 50 being in staggered relation with the tubes 34 and 36.

In FIG. 4, there is shown in greater detail a developed longitudinal cross-section of a chamber section according to the invention. In FIG. 4, there can again be seen the outer metallic shell 5 and the high-frequency winding 32, only the input connection of which is shown and designated by the reference 33; the toroidal cage formed of metallic tubes comprises the inner row of tubes 36 and the outer row, only one of the tubes of the outer row being represented, namely the tube 34 the tubes 36 of the inner row are supplied with electric cur rent and with cooling fluids by means of the input conductor 37 while the tubes 34 are supplied by means of the conductor 40 the output conductors are not illus trated the end-plate 39, which contains the fluid inlets and the electrical input and output connections, contains an electrically insulating portion 41 which is fitted on the flange 44 of the shell 5, and a collector 47 which is represented diagrammatically. The shell 5 can be made of copper and provided if necessary with a stainless steel lining 43 on the internal wall thereof.

In FIG. 4, the composite toroidal section I is joined to two sections II and VIII which are of a type in accordance with the prior art and each comprises a corrugated bellows-type enclosure (designated by the references 10 and 10 and a removable insulating shell 8 and 8 The section II is attached to the section I by means of the flange 4 which forms part of the shell 5 of section I and by means of the end-plate 12 which forms part of section II similarly, the section I is attached to the section VIII by means of the end-plate 39 which forms part of section I and by means of the flange 51 which forms part of section VIII.

In the case of FIG. 4 and for the sake of clarity of the figure, it has not been considered necessary to illustrate the protective shield for which provision may be made and which is constituted by the metallic tubes 50 of the left-hand portion of FIG. 2. The rows of tubes 34 and 36 carry an electric current in accordance with either of the alternative arrangements illustrated in FIG. 2.

The connections (fluid connections and electrical connections) are established either in series or in parallel by means of the collector 47. Pumping of the section I is carried out through the orifice 52 by means not shown in FIG. 5 but can also be carried out through one or a number of orifices located in other sections, in particular in sections II or VIII. Pumping through an orifice 52 located as shown in FIG. 4 makes it possible to obtain within the zone 54 located behind the highfrequency winding 32, a pressure of lower value than the pressure existing in the zone in which the plasma is formed. This makes it possible to pump the ionized or neutral particles which have escaped from the plasma and penetrated into said zone 54. In this case, the device according to the invention performs a function which is similar to that of the diverters employed in the prior art.

In the different modes of application of the invention which have just been described, the metallic rods (or tubes) were located along toroidal surfaces and more precisely along plane curves obtained by cutting the toroidal cage along planes at right angles to the axis of the torus. In the case of a cylindrical chamber, said metallic rods (or tubes) are located along generating-lines of the cylinder. Nevertheless, the invention is not limited to this single arrangement but also extends to an arrangement in which the rods (or tubes) would be disposed at the surface of a torus but along skew curves. More precisely, an alternative form of the invention resides in winding the rods in a helix so that the cage section which is formed should thus have a twisted configuration.

Said cage which is formed of helical tubes is shown diagrammatically in FIG. 5. The advantage of this alternative arrangement lies in the possibility of obtaining a structure which is very similar to that of the known Torsatron. To this end, it is only necessary to group the tubes 60 together in an even number of bundles, to connect the tubes of any one bundle so that they are coupled electrically in parallel and to pass an electric current through one bundle out of two, the currents which pass through the different bundles being in the same direction. In FIG. 5, there are shown three bundles 62, 64 and 66 of three tubes which carry the same current, the three bundles being separated by the tubes which do not carry a current. Under these conditons it is known that, among the magnetic surfaces which are formed, there is only one which is closed and which is capable of confining the plasma in an effective manner. In this case, only one row of tubes is sufficient to ensure protection of the shell.

This method, which consists in passing an electric current only within bundles of tubes alternately with respect to bundles which do not carry a current can be applied to the case in which the tubes are not helical and the cage is not twisted. It is thus possible to obtain particular and advantageous magnetic-field distributions other than those illustrated in FIG. 4.

The structure which is proposed according to the invention offers in addition to the features already mentioned the advantage of being adaptable to all confinement configurations which are found in chambers of the prior art, namely solenoid coils disposed along the torus, helical windings arranged along the surface of the chamber, or pulsed coils.

In the alternative embodiments hereinabove described, the shape and nature of the metallic tubes which constitute the row or rows of thecage section must be calculated so as to minimize high-frequency losses within the tubes. These losses arise from eddy currents generated within the thickness of the metallic conductors by the magnetic component of the heating high-frequency field.

Let 8 be the skin thickness as given conventionally by the relation 8 (p. p w) one-half wherein p. and p are respectively the permittivity and the conductivity of the metal constituting the tubes and m is the pulsation of the high-frequency field. Let p be the internal radius of the tube and Ap be the wall thickness of said tube.

It has been found that tubes having transverse crosssections as shown in FIG. 6 are well suited to the problem of construction of the protective cage. The characteristics of these different tubes are as follows Case A thin-walled tube (A p 5) Case B thick-walled tube 72 (A p 8) Case C thin-walled tube 74 (A p 8) and slitted along a generating-line 76 Case D thick-walled tube 78 (A p 8) and slitted along a generating-line 80 Case E two concentric tubes, the inner tube 82 being thick-walled (A p 8) and the outer tube 84 being thin-walled (A p 8). The tubes may be separated if necessary by an insulator 86.

Case F two concentric tubes, the inner tube 82 being thick-walled (A p 8), the outer tube 88 being thin-walled (A p 6) and slitted along a generating-line 90.

Case G the outer tube 88 is thin-walled and slitted along the generating-line 90 the inner tube 92 is thick-walled and slitted along the generating-line 94. The two tubes 88 and 92 are welded and the complete assembly constitutes a leak-tight tube.

Case H the inner tube 96 is thick-walled and formed of insulating material having low dielectric losses this tube is covered with a thin metallic layer 98 which is slitted along a generating-line 100.

Metals which are well suited to the construction of these tubes are copper and stainless 'steel. In the case of association of two tubes having different thicknesses (case F, for example), the metal having good conductivity (copper) preferably forms the thick-walled inner tube 82 and the metal which has lower conductivity but offers high resistance to ion bombardment (stainless steel) preferably forms the thin-walled outer shell 88.

Inasmuch as the losses are lower as the conductivity of the metal forming the core of the tube is higher, this metal is superconductive in an alternative form of the invention and the tubes carry a circulation of liquid helium. The superconductive metal is then covered with a thin layer (A p 8) of metal having low desorption.

What we claim is:

l. A chamber for the high-frequency heating of a conducting medium and in particular a plasma, said chamber being formed of assembled sections each comprising joining means and a shell section formed of leak-tight material, said chamber comprising at least one composite section which contains a high frequency winding within said shell section; and a cage section formed of a plurality of metallic rods provided between said conducting medium and said high-frequency winding, said plurality of metallic rods forming said cage section being arranged such that substantially all the particles constituting said conducting medium can bombard only a metallic surface.

tallic rods are grouped together in an even number of bundles in which one bundle out of two carries an electric current, the currents being in the same direction.

4. A chamber according to claim 1, wherein said cage section comprises two rows of metallic rods, each row having the same number of rods, one row forming an outer row and being adapted to surround the other row which forms an inner row.

5. A chamber according to claim 4, wherein the rods of said inner row are connected electrically in series and carry an electric current while the rods of the outer row do not carry electric current and are located in the magnetic shadow of the rods of said inner row.

6. A chamber according to claim 4, wherein the rods of the two rows carry an electric current, wherein the rods in each row are connected in series and each rod of the outer row is in the geometrical shadow of a rod of the inner row, the electric currents within said two rods being in the same direction.

7. A toroidal chamber according to claim 6, wherein a third row of metallic rods forming a protective shield is placed between the two rows of metallic rods which carry an electric current.

8. A chamber according to claim 1, wherein said metallic rods which form the cage section are hollow and carry a circulation of cooling fluid.

9. A chamber according to claim 8, wherein the cooling fluid is liquid nitrogen.

10. A chamber according to claim 1, wherein one section out of two is of the composite type, the shell of the nomcomposite sections being metallic.

11. A chamber according to claim 1, wherein said rods are hollow and slitted along a generating-line.

12. A chamber according to claim 1, wherein said rods contain a metallic core formed of material having good electrical conductivity.

13. A chamber according to claim 12, wherein said material of the metallic core is superconducting.

14. A chamber according to claim 12, wherein the core is covered with a layer of metal having very low desorption and wherein the thickness of said layer-is smaller than the skin thickness in the metal.

15. A chamber according to claim 1, wherein said rods support an insulating core having low dielectric losses. 

1. A chamber for the high-frequency heating of a conducting medium and in particular a plasma, said chamber being formed of assembled sections each comprising joining means and a shell section formed of leak-tight material, said chamber comprising at least one composite section which contains a high frequency winding within said shell section; and a cage section formed of a plurality of metallic rods provided between said conducting medium and said high-frequency winding, said plurality of metallic rods forming said cage section being arranged such that substantially all the particles constituting said conducting medium can bombard only a metallic surface.
 2. A chamber according to claim 1, wherein said metallic rods are wound in a helix and form a twisted-cage section.
 3. A chamber according to claim 1, wherein said metallic rods are grouped together in an even number of bundles in which one bundle out of two carries an electric current, the currents being in the same direction.
 4. A chamber according to claim 1, wherein said cage section comprises two rows of metallic rods, each row having the same number of rods, one row forming an outer row and being adapted to surround the other row which forms an inner row.
 5. A chamber according to claim 4, wherein the rods of said inner row are connected electrically in series and carry an electric current while the rods of the outer row do not carry electric current and are located in the magnetic shadow of the rods of said inner row.
 6. A chamber according to claim 4, wherein the rods of the two rows carry an electric current, wherein the rods in each row are connected in series and each rod of the outer row is in the geometrical shadow of a rod of the inner row, the electric currents within said two rods being in the same direction.
 7. A toroidal chamber according to claim 6, wherein a third row of metallic rods forming a protective shield is placed between the two rows of metallic rods which carry an electric current.
 8. A chamber according to claim 1, wherein said metallic rods which form the cage section are hollow and carry a circulation of cooling fluid.
 9. A chamber according to claim 8, wherein the cooling fluid is liquid nitrogen.
 10. A chamber according to claim 1, wherein one section out of two is of the composite type, the shell of the non-composite sections being metallic.
 11. A chamber according to claim 1, wherein said rods are hollow and slitted along a generating-line.
 12. A chamber according to claim 1, wherein said rods contain a metallic core formed of material having good electrical conductivity.
 13. A chamber accordiNg to claim 12, wherein said material of the metallic core is superconducting.
 14. A chamber according to claim 12, wherein the core is covered with a layer of metal having very low desorption and wherein the thickness of said layer is smaller than the skin thickness in the metal.
 15. A chamber according to claim 1, wherein said rods support an insulating core having low dielectric losses. 